1. Field of the Invention
This invention relates generally to oxidizing volatile organic compounds. It relates particularly to a process for oxidizing volatile organic compounds to CO.sub.2 and H.sub.2 O with minimal addition of energy. This process includes exposing a gaseous mixture containing hydrocarbons or other volatile organic compounds, and an oxidizing agent to a catalyst of a noble metal dispersed on a metal oxide possessing more than one stable oxidation state.
2. Description of Related Art
In many applications it is highly desirable, if not necessary, to remove hydrocarbons and other volatile organic compounds from the air via oxidation to CO.sub.2 and H.sub.2 O without the aid of filters and with minimal heating of the catalyst. By way of example, there has been a long-standing need for a method to remove volatile organic compounds from indoor air--i.e., breathable air in enclosed spaces such as homes, automobiles, airplanes, ships, boats, and industrial plants where there may be high concentrations of said compounds. Other significant long-standing needs include the need to purify compressed air and other oxygen-containing gases, as well as the employment of personal safety masks in the removal of volatile organic compounds from the atmosphere. There has also been a need for such a method in selective chemical sensors and catalytic converters for combustion processes, including internal combustion engines which utilize gasoline, diesel, natural gas, and alcohol fuels.
Prior methods employed to remove volatile organic compounds in such applications include: (1) the purification of enclosed spaces by filtering and exhausting to outside air, (2) the use of Hopcalite.RTM. catalyst heated to 200.degree. C.-315.degree. C. for the oxidation of alcohols, and (3) the employment of three-way noble catalysts, such as those presently used in automobiles for emissions control. These methods are unsatisfactory for several reasons. First, filters employ a surface on which the species to be removed is absorbed. The amount of said species to be removed depends on the total number of active surface sites available for absorption. Once all sites are occupied, the filter ceases to work and must be replaced. Filters contain the absorbed volatile organic compounds and may require special disposal as hazardous waste. Second, exhausting to the outside air requires duct work, fans and electrical power and adds contaminant gas to the atmosphere. Third, the Hopcalite.RTM. catalyst requires large quantities and high temperatures to accomplish oxidation of a given alcohol. Fourth, the three-way noble catalysts have had particular trouble completely oxidizing ethanol, even at temperatures above 500.degree. C. Said catalysts similarly have had trouble oxidizing methane and do not allow catalytic control of methane emissions. They require exhaust gas temperatures of 300.degree. C. and higher to convert 50% of non-methane hydrocarbons to CO.sub.2 and H.sub.2 O (light-off) and temperatures of 450.degree. C. or higher for methane light-off.
It is accordingly a primary object of the present invention to provide an efficient means for removal of hydrocarbons and other volatile organic compounds from the medium in which it exists as an unwanted, toxic impurity, without the use of filters and further with the addition of minimal energy.
It is another primary object of the present invention to provide for the removal of hydrocarbons and other volatile organic compounds from the medium in which it exists as an unwanted, toxic impurity, by presenting a catalytic process for the oxidation of hydrocarbons and other volatile organic compounds to carbon dioxide and water with the addition of minimal energy.